Turbochargers are a type of forced induction system. They deliver compressed air to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can enable the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel located in a turbine housing. The turbine wheel is solidly affixed to one end of a shaft. A compressor wheel is mounted to the other end of the shaft and held in position by the clamp load from the compressor nut. A bearing housing (1) rotationally supports the shaft. The primary function of the turbine wheel is simply to provide rotational power to drive the compressor. In EGR type engine systems a function of the turbine stage is to control the backpressure in the exhaust system to enable EGR flow to be driven, from the exhaust system, into the engine inlet system.
The power developed by the turbine stage is a function of the expansion ratio across the turbine stage. That is, the expansion ratio from the turbine inlet to the turbine exducer.
The compressor stage consists of a wheel and its housing. Filtered air is drawn axially into the inlet of the compressor cover (20) by the rotation of the compressor wheel (21). The power delivered by the turbine stage to the shaft drives the compressor wheel to produce a combination of static pressure with some residual kinetic energy and heat.
In a turbocharged engine system there is a basic mismatch between the engine output and the turbocharger output. Since an internal combustion (IC) engine is a positive displacement device the flow rate is approximately proportional to the engine speed Ne. A turbocharger is a rotordynamic device, whose characteristics are analogous to a simple throttle, and so the flow rate is, to a large extent, independent of its speed Nt. The expansion ratio across the turbocharger increases as the square of the flow rate. Because of this the turbocharger is really only ideally matched to the engine at one operating point.
From a matching standpoint, if the engine application is, for example, and on-highway truck, then the sweet spot for the match will most likely be at the engine rated point. Determining that the sweet spot for the match is at rated point will produce less favorable matching at, for example, the low engine speed operating point. This will mean that the engine will be more likely to produce more particulates in the engine low speed range than desired by the engine manufacturer (to meet emissions regulations) and the engine will not feel responsive to the driver. If, on the other hand, the turbocharger is matched to the engine low speed point, the Ne engine performance will be better, the turbocharger-engine combination will have improved transient performance and the particulates emissions will be reduced. However at Ne the turbocharger will over-boost the engine. To resolve the Ne over-boost issue, a wastegate is used to reduce the turbine power to the compressor and thus reduce the boost pressure at this engine operating point.
FIG. 2 depicts a typical map for a compressor stage. The Y axis (25) is the pressure ratio, the X axis (26) is the mass flow rate in Kg/sec. The left boundary is the surge line (21). This is a test-generated line. At each speed line, the surge point is detected, noted, then interpolated for the entire map. At the surge point, oscillatory flow behavior causes a flow blockage. In the surge condition the flow detaches from the suction surface of the blade causing instability in the flow, which oscillates as the flow attaches and detaches from the blade. The surge condition moves with installation conditions so it must be tested for each set of installation parameters. In the surge condition the turbo reacts violently and must be kept out of this operating regime.
The right boundary is the choke line (24). This line is generated by selecting a minimum value of efficiency (often 65%), on each speed line in the region where there is a steep drop in efficiency caused by the air flow reaching sonic velocity. In the choke regime, the turbo operates smoothly but the pressure ratio and efficiency fall, and temperatures rise. The nearly horizontal lines (23) are lines of equal turbocharger speed.
Line 27 is the example engine operating line. This line shows, for a given set of conditions, where the map fits the air requirements of the engine operating regime. FIG. 2 represents a turbocharger map, with the engine operating line marked on it. This would be for the case of a fixed turbocharger. By fixed, what is meant is that the turbocharger has matching limits set only by the choice of wheels and housings; there are no control devices. FIG. 3 shows the same basic map but the turbocharger is equipped with a wastegate which limits the boost to, in the case of the example, a pressure ratio of 3.43. In this case the engine operating line (28), FIG. 3 follows the engine operating line (27) of FIG. 2 until the wastegate opens, then the engine operating line curves at the point of wastegate opening to a more horizontal line, representing limited pressure ratio.
The design of the turbine stage is a compromise between the power required to drive the compressor, the aerodynamic design of the stage, the inertia of the rotating assembly, of which the turbine is a large part; the turbocharger operating cycle which affects the structural and material aspects of the design; and the near field both upstream and downstream of the turbine wheel with respect to blade excitation.
Engine boost requirements are the predominant driver of compressor stage selection. The selection and design of the compressor is a compromise between the boost pressure requirement of the engine; the mass flow required by the engine; the efficiency required by the application; the map width required by the engine and application; the altitude and duty cycle the engine is to be subjected; the cylinder pressure limits of the engine; etc.
There are many specifics outside of the turbocharger which cause there to be limits placed upon the turbocharger. Limitations of the engine, such as cylinder pressure limits may mean that the boost level needs to be held below a maximum allowable pressure at any point in the engine operating regime. Altitude limits may generate the need to control turbocharger speeds for structural reasons. Marketing forces may cause there to be a need for a modifiable boost level control.
There are engine limiting factors, within the events of combustion, both internal and external to the combustion chamber. Some of these factors are: the intake of the air charge; the compression of the air charge; the expansion and exhaust strokes of the engine; the compression ratio; the injection of fuel; the shape, timing and nature of the injection plume; the ignition of fuel; and the characteristics of the ignition, be it initiated by compression or spark; the location and design of the sealing rings; the design of the piston crown and cylinder head; basin, bowl bump, flat, hemispherical, swirl initiating, non swirl, stratified, homogeneous; the air-fuel ratio; etc. The cylinder pressure limits are usually controlled by features in the engine design such as exhaust sealing methods and materials, valve, and valve seat materials, piston design, piston ring design, cylinder temperature, cylinder head cooling and structural limitations, heat, transfer rates and knock in spark ignition engines. As mentioned above, the cylinder pressure limits are a predominant driver of the need for wastegates.
The inertia of the rotating assembly is predominantly the inertia of the turbine wheel. The moment of inertia is the sum of the separate inertias taken at distinct slices through the part.The Moment of Inertia I=ΣM·R2                 Where M is the mass of the section in question and        R is the radius of the section.        
The reason this is important to turbocharger operation is that the addition of a wastegate to the turbine stage allows matching to the low speed range, with a smaller turbine wheel and housing and thus the addition of a wastegate brings with it the option for a reduction in inertia. Since a reduction in inertia of the rotating assembly typically results in a reduction of particulate matter (PM). Wastegates have become common in on-highway vehicles. The problem is that most wastegates are somewhat binary in their operation which does not fit well with the linear relationship between engine output and engine speed.
As explained above there are many reasons for incorporating wastegates:                1. To limit maximum cylinder pressure, the wastegate can be a simple device which simply limits boost and thus the mass flow of air provided to the combustion chamber.        2. To limit maximum cylinder temperature the wastegate can be used as a simple device to limit the air mass flow intake and thus the A/F ratio and combustion temperature.        3. To allow marketing to change power setting of engines. By simply changing the seat pressure of the actuator spring, at the time of assembly of the turbocharger, the point at which the wastegate begins to open can be relatively easily changed, with minimal part number changes.        4. To fine tune the power settings of the engine. When an engine is assembled there exists a range of variables, in all parts of the engine, due to tolerances build up, which change the power produced by the engine. Since Diesel engines are sold by fractional power ratings eg 112 kW, 130 kW, 142 kW, 149 kW, the use of the infinite variation of set points available with a wastegate is a powerful fine-tuning tool. For example when an engine first goes to production, the tolerances of all the production components, differentiated from the prototype components, often produces an engine with different power and torque. Instead of having to change components to make the target settings, the wastegate setting can just be altered quickly, thus not delaying production of a certified engine. The problem with this “easy fix” is that since the simple flat faced valve and seat design does not modulate flow well with valve opening angle the operation of the wastegate can be coarse.        
Wastegates come in two basic configurations; a poppet type valve and a swing valve type. FIG. 4A depicts a typical swing valve wastegate in which the valve (31) is affixed to a valve arm (33) such that the valve is free to move so that it seals on a seat (32) machined into the housing (30). The housing is typically a cast iron or steel turbine housing or exhaust manifold through which flows exhaust gas (35) upstream of the turbine wheel. Machined into the housing is a pivot (34) about which the valve arm (33) may rotate. When the wastegate is commanded closed, the valve arm (33) is driven to the closed position by an actuator mechanically connected to the valve arm (33) such that the valve (31) seals on its seat (32) and no exhaust gas can escape from the housing (30).
When the wastegate is commanded to open (FIG. 4B) the actuator moves such that the valve arm (33) rotates to position (33A) allowing exhaust gas (35) to flow both through the valve orifice in a direction (35B) out of the housing (30) with the remaining exhaust gas (35A) free to travel still in the housing (30). Thus the flow (35) through the housing with the wastegate valve closed is greater than the flow (35A) through the housing with the wastegate valve open. This reduces the energy available to drive the turbine wheel.
FIG. 5A depicts a typical poppet valve wastegate in which the valve (41) is guided in a housing (48) by a sleeve (47) such that the valve is free to move perpendicular to its seat, such that it seals on a seat (42) machined into the housing (40). The housing is typically a cast iron or steel turbine housing or exhaust manifold through which flows exhaust gas (45) upstream of the turbine wheel. The wastegate housing (48) typically mounts a pneumatic diaphragm which can drive the valve (41) by either pressure or vacuum, depending upon the engine choice. When the wastegate is commanded closed, the valve (41) is driven to the closed position by the actuator mechanically connected to the valve (41) such that the valve (41) seals on its seat (42) and no exhaust gas can escape from the housing (40).
When the wastegate is commanded to open (FIG. 5B) the actuator moves such that the valve (41) is lifted off its seat (42) to position (41A) allowing exhaust gas (45) to flow both through the valve curtain in a direction (45B) out of the housing (48) through orifices cut in the housing to allow the gas (46) to escape to atmosphere, with the remaining exhaust gas (45A) free to travel still in the housing (40). Thus the flow (45) through the housing with the wastegate valve closed is greater than the flow (45A) through the housing with the wastegate valve open. This reduces the energy available to drive the turbine wheel. Since the escaping exhaust gas can only flow through the cylindrical orifice between the valve head and the valve seat, the flow change for a given valve displacement is not very linear.
FIG. 7 depicts the invention in its most simple form. The valve top (31), which affixes the valve to the valve arm (33) is as before. The lower part of the valve (90) is modified to take the shape of a horn. The valve opens in the same manner as before (FIG. 4A, 4B) but now the airflow from the duct flows in the direction (35) around the horn (90) so that the escaping air flows in the annular orifice between the outside surface of horn (90) and the inside surface of the port (37).
By incorporating smart controls and feedback on the valve position, this crude motion can be tailored to a finer modulation. But that adds significant cost to the engine since the electronics must live in a hostile environment of exhaust gas at approximately 860° C. to 1050° C. and vibration, both of which are hostile environments for electronics. Thus there is a need for a simple, finely modulated exhaust gas control.